Virtually Mapped Stacks: Do not disable interrupts

[Christoph Lameter]

The reason to disable interrupts seems to be to avoid switching
to a different processor while handling per cpu data using
individual loads and stores. If we use per cpu RMV primitives
we will not have to disable interrupts.

Signed-off-by: Christoph Lameter <c...@linux.com>

Index: linux/kernel/fork.c
===================================================================
--- linux.orig/kernel/fork.c
+++ linux/kernel/fork.c
@@ -205,19 +205,17 @@ static unsigned long *alloc_thread_stack
        void *stack;
        int i;

-       local_irq_disable();
        for (i = 0; i < NR_CACHED_STACKS; i++) {
-               struct vm_struct *s = this_cpu_read(cached_stacks[i]);
+               struct vm_struct *s;
+
+               s = this_cpu_xchg(cached_stacks[i], NULL);

                if (!s)
                        continue;
-               this_cpu_write(cached_stacks[i], NULL);

                tsk->stack_vm_area = s;
-               local_irq_enable();
                return s->addr;
        }
-       local_irq_enable();

        stack = __vmalloc_node_range(THREAD_SIZE, THREAD_SIZE,
                                     VMALLOC_START, VMALLOC_END,
@@ -245,19 +243,15 @@ static inline void free_thread_stack(str
 {
 #ifdef CONFIG_VMAP_STACK
        if (task_stack_vm_area(tsk)) {
-               unsigned long flags;
                int i;

-               local_irq_save(flags);
                for (i = 0; i < NR_CACHED_STACKS; i++) {
-                       if (this_cpu_read(cached_stacks[i]))
+                       if (this_cpu_cmpxchg(cached_stacks[i],
+                                       NULL, tsk->stack_vm_area) != NULL)
                                continue;

-                       this_cpu_write(cached_stacks[i], tsk->stack_vm_area);
-                       local_irq_restore(flags);
                        return;
                }
-               local_irq_restore(flags);

                vfree_atomic(tsk->stack);
                return;